Integrated circuits are very complex devices that include multiple layers. Each layer may include conductive material, isolating material while other layers may include semi-conductive materials. These various materials are arranged in patterns, usually in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits.
Integrated circuits are manufactured by complex multi-staged manufacturing processes. During this multi-staged process resistive material is (i) deposited on a substrate/layer, (ii) exposed by a photolithographic process, and (iii) developed to produce a pattern that defines some areas to be later etched.
Various metrology, inspection and failure analysis techniques evolved for inspecting integrated circuits both during the fabrication stages, between consecutive manufacturing stages, either in combination with the manufacturing process (also termed “in line” inspection techniques) or not (also termed “off line” inspection techniques). Various optical as well as charged particle beam inspection tools and review tools are known in the art, such as the Elite™, UVision™, Complus™ and SEMVision™ of Applied Materials Inc. of Santa Clara, Calif.
A typical inspection tool includes multiple sensor that convert received light to an electrical signal, such as to enable electrical components such as image processors to process the receives signals and to evaluate the stat of an inspected object.
Two commonly used sensors include the reverse biased solid state photo diode (PD) and the avalanche photo diode (APD). These diodes convert received light signals to current. Typically, these diodes are biased in a reverse voltage that can affect their characteristics.
The terminal capacitance of the biased solid state photo diodes is responsive to the reverse voltage while the gain of the avalanche photo diodes is responsive to the reverse voltage.
FIG. 1 illustrates a prior art multiple sensor unit 10. Multiple sensor unit 10 includes multiple (N) sensors. For convenience of explanation only it is assumed that N=9. The multiple sensor unit 10 can include biased solid state photo diodes or the avalanche photo diodes. For simplicity of explanation these diodes are referred to as sensing diodes.
Multiple sensor unit 10 includes multiple sensing diodes D1-DN 51-59. The anode of all these diodes is connected to the same electrode (at node A0 30).
The cathodes of diodes D1-DN 51-59 are connected (at output nodes A1-AN 41-49) to load resistors R1-RN 61-69. When light impinges on a certain sensing diode Dn then Dn outputs a current In that is translated to a voltage drop Voutn=In*Rn. This voltage is the output of the multiple sensor unit 10.
FIG. 2 illustrates another prior art multiple sensor unit 20. Multiple sensor unit 20 of FIG. 2 differs from the multiple sensor unit 10 of FIG. 1 by including a trans-impedance amplifier instead of a load resistor. This configuration is less noisy.
FIG. 2 illustrates two trans-impedance amplifiers, although each sensing diode is connected to its own trans-impedance amplifier. The first trans-impedance amplifier is connected to the cathode of D1 51 and includes an operational amplifier 71 that includes a positive input 171, a negative input 271 and an output 371. A resistor R1 81 is connected between the positive input 171 and the output 371. The negative input 271 is grounded so that the positive input 171 is virtually grounded. The current that flows through sensing diode D1 51 is converted to an output signal at 371.
The second illustrated trans-impedance amplifier is connected to the anode of DN 59 and includes an operational amplifier 79 that includes a positive input 179, a negative input 279 and an output 379. A resistor RN 89 is connected between the positive input 179 and the output 379. The negative input 279 is grounded so that the positive input 179 is virtually grounded. The current that flows through sensing diode DN 59 is converted to an output signal at 379.
The sensing diodes within a multiple sensor unit slightly differ from each other. The differences difference between the different sensing diodes is also known as sensing diode non-uniformity. This non-uniformity introduces errors in the detection and thus needs to be reduced. The prior art configurations of FIG. 1 and FIG. 2 are not adapted to compensate for these non-uniformities as all the sensing diodes are placed at the same reverse voltage.
There is a need to provide an efficient manner to control the characteristics of multiple sensors.